1. Field of the Invention
The invention relates to a method for calibrating a radius test bench for measuring radii of optical elements, in particular of lenses and spherical mirrors.
2. Description of the Related Art
So-called radius test benches are known for testing the radii of curvature of spherical lenses and spherical mirrors. In this case, the sample is placed in reflection in a spherical wave emitted by an interferometer. In order to check the radius of curvature, the sample is positioned once in an autocollimation position and once in the cat's eye position. This procedure is described in detail in an article by Lars A. Selberg: “Radius measurement by interferometry” published in Optical Engineering 31(9) September 1992 pages 1961-1966 which is herewith incorporated by reference. The cat's eye position is also described in the well-known textbook “Optical Shop Testing”, 2nd edition, edited by Daniel Malacara; 1992 John Wiley & Sons Inc. In this position a mirror, which may be of arbitrary shape, e.g. plane or spherical, is located in the focus of a convergent spherical wave, so that this wave runs back into itself. The rays impinge on the mirror not perpendicularly. This is a fundamental difference to the autocollimation position. In the latter the rays impinge everywhere perpendicularly on the mirror and are reflected into theirselves. In both positions an incident spherical wave runs back into itself and it is the use of both positions which enables the construction of a radius test bench of the related art. The spacing of the two positions is then the radius of curvature being sought. A precondition for measuring as exactly as possible is, however, a very accurate radius test bench or knowledge of unavoidable errors of the radius test bench, in order to be able to take these into account later when measuring the sample.
Such errors are discussed in the article cited above.